1. Field of the Invention
This invention relates to nuclear magnetic resonance and more particularly to a temperature compensated nuclear magnetic resonance apparatus and method.
2. Description of the Related Art
To obtain hydrocarbons such as oil and gas, a drilling assembly (also referred to as the "bottom hole assembly" or the "BHA") carrying a drill bit at its bottom end is conveyed into the wellbore or borehole. The drilling assembly is usually conveyed into the wellbore by a coiled-tubing or a drill pipe. In the case of the coiled-tubing, the drill bit is rotated by a drilling motor or "mud motor" which provides rotational force when a drilling fluid is pumped from the surface into the coiled-tubing. In the case of the drill pipe, it is rotated by a power source (usually an electric motor) at the surface, which rotates the drill pipe and thus the drill bit.
Bottom hole assemblies generally include several formation evaluation sensors for determining various parameters of the formation surrounding the BHA during the drilling of the wellbore. Such sensors are usually referred to as the MWD sensors. Such sensors traditionally have electromagnetic propagation sensors for measuring the resistivity, dielectric constant, water saturation of the formation, nuclear sensors for determining the porosity of the formation and acoustic sensors to determine the formation acoustic velocity and porosity. Other downhole sensors that have been used or proposed for use include sensors for determining the formation density and permeability. The bottom hole assemblies also include devices to determine the BHA inclination and azimuth, pressure sensors, temperature sensors, gamma ray devices, and devices that aid in orienting the drill bit in a particular direction and to change the drilling direction. Acoustic and resistivity devices have been proposed for determining bed boundaries around and in some cases in front of the drill bit. More recently, nuclear magnetic resonance (NMR) sensors have gained extreme interest as MWD sensors as such sensors can provide direct measurement for water saturation porosity and indirect measurements for permeability and other formation parameters of interest.
NMR sensors utilize permanent magnets to generate a static magnetic field in the formation surrounding the MWD tool. A radio frequency (RF) coil disposed between the magnets or around the magnets induces an RF magnetic field. The magnets and the RF coils are positioned so that the static and the RF fields are perpendicular to each other at least over a portion of the formation surrounding the NMR tool where the static field has a substantially uniform strength. This region is the region of interest or region of investigation. The NMR measurements corresponding to such region are needed to determine the formation parameters of interest. The NMR sensors should be designed so that region of investigation is constant, i.e., that the size and shape of this region remains the same throughout the investigation. However, the wellbore temperature increases with the depth. As the temperature increases, the permanent magnets lose strength, which reduces the size of the region of investigation and also moves it closer to the wellbore. At relatively high temperature wellbore depths, the region of investigation can overlap a part of the wellbore, which can severely affect the formation measurements due to the fluid in the wellbore. Thus, it is desirable to provide a system that will compensate for the reduction in the magnetic strength as a function of temperature.
The present invention provides MWD tools wherein the distance between opposing permanent magnets is adjusted as a function of temperature to so as to maintain at least a portion of the region of investigation substantially at constant distance from the tool body.